Method of preparing photocollographic printing plates



Patented Dec. 24, 1940 PATENT OFFICE METHOD OF PREPARING PHOTOCOLLO- GRAPHIC PRINTING PLATES James MacBride, San Francisco, Calif.

No Drawing. Application September 14, 1940, Serial No. $6,864

2 Claims.

This invention relates to improvements in processes for forming printing plates and has I particular reference to that type of process generally called the collotype process, which may be briefly described .as a method of producing images or prints, using printer's -ink, from a printing surface of gelatin, and thereby rendering a photographic quality to the print derived from the plate. 1

The principal object of my invention is to produce a printing plate which will reproduce the tones and details of a negative transparency, such as a photographic negative. I

A further object is to produce a printing plate having the mentioned properties without the use of any sort of screen,such as is used in other photomechanical half-tone processes.

A still further object is to produce a printing plate at far less cost than printing plates produced by etched metalprocesses.

Another object is to produce a printing plate which can be easily produced and which will not require expensive equipment.

A 'still further object is to produce printing plates which may be duplicated with the same degree of printing quality.

A further object is to produce printing plates of the edges of the plate will be unnecessary to secure clean or white margins in the print.

A further object is to produce a plate wherein the same may be used in printing upon practically any type or grade of paper, which cannot be done with the ordinary collotype plate.

A further object is to provide means whereby several collotype plates constructed in accordance with my invention may be used successively to produce natural color prints.

Other objects and advantages will be apparent during the course of the following description.

In the production 02 collographic printing plates the customary procedure has been to prepare a glass plateby grinding the surface and applying thereto a thin coat of albumen or gelatin to serve as a substratum for the printing surface consisting of gelatin with which has been mixed a quantity of potassium or ammonium dichromate.

This mixture is applied to the substratumed glass plates, which are supported perfectly level in a device, known as an oven. This oven must also protect the plates from vibration, drafts, and light during the process of drying, which is accomplished by dehydrating the plates by the agency of heat at approximately 120Fahrenheit,

and the temperature must remain practically constant. The plates must be allowed to become cool before the oven is opened, and the prepared plates do not keep for. more than a few days. From three to five hours may be required for the plates to dry and cool.

When such a plate is exposed to light through a negative, a visible image is formed together with a tanning or hardening action on the gelatin coat which is directly proportional to the amount of light acting at a particular point. The plate is then washed to cause the gelatin to swell and also to remove the chemicals. The plate is then dried. When dry, it is again wetted with a solution of glycerine and water to which is sometimes added such items as sodium chloride to assist the hygroscopic qualities, ammonia to help the pene tration of the glycerine solution into the coating, and oxgall to assist in getting bright prints. It is a common fault of the process that the unprinted margins of the plate end to take. ink, and it is usually necessary to mask these edges during the inking process and to remove it when the impression is being taken.

The plates do not usually have a long life-500 to 1000 prints being about all that can generally be expected from a plate.

Further, platesmade by this method render impracticable the use of coated papersyas the unprinted edges tend to be tacky or sticky, causing the paper to cling and tearing the sheet or pulling off the coating of the paper. Duplication of plates is difflcult. Close control of the size of the printing grain is not practicable. I

With my process the faults above mentioned are eliminated, and the objects first mentioned are obtained.

In producing my plates I first employ a fiat sheet or plate, which may be of any material having the proper characteristics, such as flatness, hardness, freedom from surface irregularities, and capable of maintaining these qualities during the process and use. I have found that one of the best materials to use as a base is copper, such as used by photo-engravers and for chemical means to secure maximum adhesion of the surface coat. Scrubbing with pumice powder I then plate the copper by using a simple silvering solution, orhave it electroplated with nickel or silver. This overcomes the defect of copper to harden the coating by chemical reaction.

A plate so prepared is ready to receive its colloid coating. No substratum is required.

The colloid, gelatin, is now prepared, and I have found that if grams of gelatin is melted in 100 cubic centimeters of water, a fluid state of such consistency is obtained that it does not easily flow off the edges of the plate when coating, and it sets at a satisfactory rate and dries in a relatively short time.

To this liquid gelatin I add a hardening agent, such as a 2% solution of .chrome alum in the proportion of 5 .c. c. to 100 c. c. of gelatin. However, the proportion may be varied if desired. This imparts to the gelatin a hardness which enables it to wear well in printing .and helps to eliminate tackiness.

The quantity of gelatin to be applied to the base varies. As an example, gelatin prepared as above in the quantity of .5 cubic centimeters per square inch of plate surface is very satisfactory for plates to be printed in a press not equipped with a damping device.

This gelatin is poured on the plate and guided to the edges by any convenient method and allowed to set. Theplate at this time is supported on a level surface or other means for holding the plate level.

After the gelatin has set, drying is completed in a current of warm air, requiring approximately two hours. The plate may be further processed at once or kept for any reasonable time, as it is not now sensitive to light.

When it is desired to use the plate, it is rendered sensitive to light by immersing it in a solution containing one of the alkali dlchromates, potassium, sodium, or ammonium dic omates all being satisfactory. Potassium di omate gives the greatest contrast, sodium and-ammonium dichromates give less.

r The contrastsof the printing plate are further and principally controlled by varying the concentration of the sensitizing solution and the rate of drying the plate after immersion in the sensitizing bath.

I have found that a coated base prepared as above mentioned and immersed for five minutes at 55 to 65 Fahrenheit in a bath composed of 5 grams of ammonium dichromate in 100 cubic centimeters of water will produce a plate of excellent quality.

The plate is now drained and dried in a current of air in such a manner that the time of drying may be controlled, as the time of drying directly affects the characteristics of the plate;

short drying times produce a morecontrasting printing plate than longer times. I have found thirty minutes to be a good drying time for plates 'brilliancy or softness of theprint, (3) a plate same negative on print-out proof paper as commonly used by photographers. I, therefore, use this method as a reliable means for gauging the exposure time for the printing plate, thus saving time, trouble, and material. 5

The effect of light on the sensitive surface is to harden or tan the gelatin in proportion to the strength of the light and the time for which it acts.

The plate is now immersed in a water bath, and the exact temperature of the water bath that is necessary is determined by the results desired. The gelatine swells as the water is absorbed by the gelatin. The tanned portion of thesurface is not able toswell as rapidly as the gelatin immediately under it and accommodates itself to the faster swelling under layer by cracking, or reticulating, causing the surface to have a slight roughness or tooth, called the printing grain. The rate of swelling determines whether the grain will be coarse or fine. "Slow swelling produces fine grain, while rapid swelling produces a coarse grain. Consequently, I can definitely control the grain.

The graining is completed when the plate has absorbed water to its capacity, and further immersion results only in the diffusing out of the gelatin of the chemicals used in sensitizing. These chemicals can be removed by prolonged soaking or more rapidly in running water.

This plate may now be dried and further processing completed at a later timefw it may .at once be placed in or covered with a hygroscopic fluid which is absorbed by the gelatin. This imparts to the plate the property of attracting mois- 35 ture to itself, thus maintaining itself in a damp condition.

It is here stated that. it is very important to employ a non-glycerine base hygroscopic fiuid which will not attack the gelatin and cause the same to be tacky. I, therefore, immerse the plate, dry or wet, in the hygroscopic fluid for about two hours, after which it may bewiped free of excess fluid and placed in the press for printing.

It will be found that the plate takes ink readily and that after the pressure is applied, the print therefrom will have clean white margins and clear sharp details, and the prints may be made upon a variety of papers, including some not practicable with plates produced by oven methods.

In brief my process consists of the following steps: Using metal plates, plating or silvering the plates, coating with prepared gelatin, drying the coat, sensitizing by immersion, exposure to light through a negative, swelling gelatin to form grain in exposed parts, washing, and soaking in hygroscopic fluid to make it self-damping or to facilitate mechanical damping.

It will be se-n that I have eliminated certain disadvantages, such as, (1) the operation of applying a substratum to the base, (2) 'the use of fragile plates, (3) any element of uncertainty of results, and (4) the use of a special type of drying oven.

' It will be, seen that I have in addition secured the following advantages: (1) simple procedure and equipment, (2) a measure of control of the which may be used on almost any press, (4) a plate improved in hygroscopic qualities, (5) a pla e of improved ink rejecting-qualities, ,(6) a plate which positively produces clean margins" without the use of any special device or treat-' 76 free from tacklness, (11) a plate ment, (7) a definite control oi the printing grain, (8) a more durable plate, (9) a plate that may be run at higher speeds, (10) a plate that is that will print on a wider variety of papers, and (12) a reliable meansof gauging the exposure time. It is to be understood that the form of my invention herewith shown and described is to be taken as a preferred example of the same and that various changes relative to the material, size, shape and arrangement of parts may be resorted to without departing from the spirit of the invention orthe scope of the subjoined claims.

. Having thus described my invention, I claim: 1. The process of producing a photographic printing plate consisting in coating 9. plated base with a hardened gelatin, allowing thecoating to dry, bathing the coated base in a solution con- 20 taining a sensitizing substance, allowing thecoated base to dry at a controlled rate, exposing the sensitized coated base to a photographic impression from a light illuminated negative, subjectprinting plates, which consists in coating a multilayer base with a layer of colloid substance, thereafter drying said layer, next rendering said dry layer light-sensitive by immersing the same in a solution of an alkali dichromate, next drying said sensitized layer and controlling the drying time to automatically control the printing contrasts of the plate, next exposing said sensitized dried layer under alight illuminated negative, and finally immersing said sensitized dried and exposed layerin a solution, and in this operation controlling the coarseness or fineness oi the printing grain by varying the rate of swelling of the colloid layer.

JAMES MscBRIDE. 

